This invention relates to the art of controlling magnetic flux within magnetic bodies. A switched voltage applied to a conductive winding causes the voltage induced in the winding to have such waveform and magnitude that the integral over time of the induced voltage correlates to desired changes of the magnetic induction level.
The present invention is a development of the invention disclosed in U.S. Pat. No. 6,522,517 to Edel, issued Feb. 18, 2003. This patent (U.S. Pat. No. 6,522,517) is hereby incorporated by reference, in its entirety, into this disclosure. It will be referred to herein as “the 517 patent.”
The 517 patent discloses how a varying voltage, produced by a “controllable voltage device,” may be applied to a conductive winding that magnetically interacts with a magnetic body. The varying voltage controls the voltage induced in the winding in such a way that the integral over time of the induced voltage correlates to desired changes of the induction of the magnetic body. (As used in this disclosure, the term “induction level” is synonymous with the terms “magnetization” and “magnetic flux density”). The induction level may be controlled in several ways:    (a) The induction level of a magnetic body may be caused to transition from a known induction level to a preferred induction level. (A preferred induction level of zero may be chosen to demagnetize a magnetic body).    (b) When the induction level is not known, a preferred induction level may be established by changing the induction level of the magnetic body from an unknown induction level to a known induction level and then to the preferred induction level.    (c) A preferred induction level may be maintained by causing the induced voltage across the winding to have an average value near zero (or by causing the integral of induced voltage to not exceed a predetermined value).    (d) A preferred induction level may be more strictly maintained by causing the induced voltage across the winding to continuously be near zero volts, thereby reducing the amount that the induction level fluctuates.    (e) The induction level may be made to vary with time in a preferred manner, including matching a control signal that is proportional to a reference induction level.
The varying voltage may be generated directly by an active voltage source, or the varying voltage may be generated indirectly, such as by current transformer secondary current flowing through an adjustable impedance. The key elements are a magnetic body, a conductive winding that magnetically interacts with the magnetic body, and a means of causing the induced voltage to have the appropriate waveform and magnitude.
A conductive winding that is utilized for controlling induction level may be a permanent winding that is also used for other purposes, or it may be a dedicated winding (permanent or temporary) provided solely for the purpose of controlling induction level.
These principles are most readily applied to magnetic bodies that are configured to have a relatively uniform magnetic path, such as the magnetic cores of current transformers. The accuracy of a current transformer may be improved in three ways:    (a) By demagnetizing a current transformer, inaccuracies associated with core magnetization are removed. A demagnetized current transformer can accurately measure d-c current and a-c current that has a d-c component.    (b) By keeping the integral over time of induced voltage near zero, a current transformer is better able to measure unsymmetrical currents without quickly transitioning to saturation.    (c) By reducing the amount that the induction level in the core fluctuates, inaccuracies associated with magnetizing current may be greatly reduced and the accuracy of a current transformer may be greatly improved.
The present invention utilizes voltage pulses to control the magnetization of a magnetic body, rather than a continuously variable voltage (as discussed in the 517 patent). Some background information about how pulsed voltages have been used previously in current-sensing applications will now be presented.
U.S. Pat. No. 3,768,011 to Swain, entitled “Means for Measuring Magnitude and Direction of a Direct Current or Permanent Magnet, Including Clip-On Direct Current Sensing Inductor,” enables non-contact DC current measurement utilizing an ordinary clip-on current transformer arrangement. The measurement of DC current is accomplished by continually driving a current transformer magnetic core back and forth between opposite polarities of saturation. The polarity of the voltage pulse is toggled based on the secondary current reaching a magnitude that is sufficient to ensure saturation. The secondary current signal is filtered to remove the saturating peaks, and the filtered signal is representative of the magnitude of the primary current. In this patent, the voltage pulses are simply used to drive a magnetic core to saturation, not to control the magnetization of the core at a level other than saturation. Measurement is limited to DC current.
U.S. Pat. No. 4,274,051 to Condon, entitled “Electromagnetic Arrangement for Measuring Electrical Current,” is directed toward the measurement of DC current in telephone systems. Two magnetic cores are utilized, along with multiple windings on each core and a pulsed control signal. Saturating pulses are applied to both cores, and differences in sensed voltages across other windings are used to enable measurement of DC current. In this patent, the voltage pulses are simply used to drive the magnetic cores to saturation, not to control the magnetization of the cores at a level other than saturation.
U.S. Pat. No. 4,278,939 to Henry, entitled “Electromagnetic Arrangement for Measuring Electrical Current,” claims to be an improvement of the U.S. Pat. No. 4,274,051 patent listed above. Saturating pulses are applied to both cores, and differences in sensed voltages across other windings are used to determine a DC component. In this patent, the voltage pulses are simply used to drive the magnetic cores to saturation, not to control the magnetization of the cores at a level other than saturation.
U.S. Pat. No. 4,456,875 to Chenier, entitled “Demagnetization Circuit for Current Transformers” applies switched voltages to current transformers to demagnetize them more rapidly and with less power than by utilizing more conventional methods. However, this invention demagnetizes current transformers only while they are out of service. This invention can not be used to control the magnetization of a current transformer core while it is being acted on by other magnetomotive forces (such as a primary current).
U.S. Pat. No. 4,914,383 to Wilkerson, entitled “Non-Contact Ammeter,” is another invention that uses voltage pulses to enable non-contact DC current measurement with a magnetic core. This patent discloses alternate control means for controlling the polarity of the voltage pulses. Rather than sensing peak secondary current as an indicator of saturation (to alternate the voltage pulse polarity), this patent uses either a hall-effect sensor embedded in a notch in the core, or a sense winding to sense saturation. With a sense winding, saturation is indicated when the induced voltage drops to zero, at which point the polarity of the voltage pulse is switched. With the hall-effect sensor in a notch, saturation is indicated by a sudden increase of magnetic flux in the notch, which occurs when the unnotched part of the core becomes saturated. Apart from the means of sensing saturation, this invention is similar to others that continually drive a magnetic core back and forth between opposite polarities of saturation. Voltage pulses are simply used to drive the magnetic core to saturation, not to control the magnetization of the core at a level other than saturation.
U.S. Pat. No. 5,008,612 to Otto, entitled “Current Sensor,” measures DC and wide-band AC currents by utilizing two distinct transformers in combination. The measurement of a DC current component is accomplished by continually driving a first current transformer magnetic core back and forth between opposite polarities of saturation (similar to many other prior-art DC current sensors). A second transformer is added to measure AC current components. This second current transformer is maintained in a non-saturated state by balancing the DC component of the primary current with an equivalent and opposite DC component in the secondary circuit (as determined using the first transformer). Thus, the second current transformer operates in the usual mode for AC current transformers, and its frequency response is limited only by the usual magnetic core considerations. The result is an AC/DC current sensor having a relatively large frequency response. However, there is a fair amount of noise coupled to the primary circuit due to the cyclic saturation of the first transformer, and the combination of two transformers and associated circuitry tends to be complex and expensive. The patent explains how the noise coupled to the primary may be mitigated by using a third transformer, but this adds still more to the expense and complexity of the current sensor. As in other prior-art DC current sensors, voltage pulses are simply used to drive the transformer core to saturation, not to otherwise control the magnetization of the transformer core.
U.S. Pat. No. 5,053,695 to Canter, entitled “DC Current Monitor,” is another invention that purposely saturates a magnetic core to enable non-contact DC current measurement. “A reset current is caused to periodically flow through the secondary coil which produces a magnetic flux oppositely polarized to the flux created by the current in the primary, thus allowing ongoing measurements to be made.” The reset current is of such magnitude to ensure saturation. The secondary current is sampled after the reset pulse and after the primary current causes the magnetization of the magnetic core to transition away from saturation. Though this patent speaks of a “reset current” rather than a voltage, the effect of transitioning the core to saturation is similar to other patents. As with previous patents, the magnetic core is purposely driven to saturation, but the magnetization of the core is not effectively controlled otherwise.
U.S. Pat. No. 5,293,121 to Estes, Jr., entitled “Isolated Current Monitoring Circuit for Measuring Direct and High Duty Factor Currents,” is another invention that uses voltage pulses to enable non-contact DC current measurement. In this patent, a current transformer is periodically driven to saturation during a first time interval and brought out of saturation during a second time interval. The secondary current is measured during the second time interval. This reduces the amount of noise induced on the primary circuit, relative to devices that drive the transformer core to alternating states of saturation. This invention appears to be applicable only to measuring unipolar DC currents. While, this invention effectively moves the operating point of a magnetic core to a non-saturated state, it does not effectively control the magnetization of the core while it is not saturated.
U.S. Pat. No. 5,811,965 to Gu, entitled “DC and AC Current Sensor Having a Minor-Loop Operated Current Transformer,” continuously applies voltage pulses of alternating polarity to a current transformer while it is in service. Like the U.S. Pat. No. 5,811,965, a current transformer core is periodically driven alternately between a saturated state and an unsaturated state. One improvement over the U.S. Pat. No. 5,811,965 patent appears to be that bipolar current measurement is possible, rather than just unipolar measurement. Secondary current is measured at the end of each voltage pulse, and “the sample having the lower absolute value is selected as a sample proportional to line current.” In this patent, voltage pulses are used to drive the CT away from saturation, but the magnetization of the CT core is not otherwise controlled so as to remain at a preferred induction level. The pulses merely drive the CT into and out of saturation every two-pulse cycle.